A 16-year-old male presents with a nagging, worsening cough that has been present for 4 weeks. His blood smear is shown here. What is the diagnosis?

unknown 24
A. Bordetella pertussis
B. Streptococcus pharyngitis
C. Mycoplasma pneumoniae
D. Respiratory syncytial virus





(Scroll down for the answer)








The answer is A, Bordetella pertussis infection. Bordetella pertussis infection is one of only two infectious diseases (the other is infectious lymphocytosis) that manifests in the blood as a mature lymphocytosis. A “mature” lymphocytosis is one in which the cells appear relatively small, with condensed chromatin, and no unusual “reactive” changes – basically, they look like regular old mature lymphocytes.

If you see a mature lymphocytosis in an adult, by the way, you need to rule out chronic lymphocytic leukemia, which can look very similar to a benign, mature lymphocytosis. In fact, you really can’t tell the two apart reliably just by looking at a blood smear. You need to do flow cytometry to be sure. In CLL, the cells are positive for B cell markers but also positive for CD5. In a benign lymphocytosis, the cells would be a mixture of B and T cells – and you wouldn’t see that CD5 marker on the B cells. CLL doesn’t occur in kids – so it’s not a consideration in this patient.

Back to our case. We talked about a mature lymphocytosis, which is what you see in Bordetella pertussis infection and in infectious lymphocytosis. Other viral infections lead to a “reactive” lymphocytosis. In this type of lymphocytosis, the lymphocytes don’t look small and mature. They tend to look weird – they have voluminous cytoplasm, for example, or large nuclei with fine chromatin and nucleoli.

There’s one other clue to Bordetella that people don’t talk about much, but it’s pretty cool.   In Bordetella, the lymphocytes often have a weird “clefted” appearance. There are several clefted lymphocytes in the slide above. Here are two of them up close:

unknown 24a

This is a good little finding to tuck in the back of your brain. If you see it, and you diagnose Bordetella from the blood smear, people will think you’re a genius!

If you liked this case, and want to test yourself with other unknown cases, here are some to try:

  • Case 1: 20-year-old male who died suddenly
  • Case 2: 72-year-old male with right calf mass
  • Case 3: 67-year-old female with pancytopenia
  • Case 4: 59-year-old male with severe headaches
  • Case 5: 38-year-old female with deep venous thrombi
  • Case 6: 13-year-old male with cerebellar mass
  • Case 7: 45-year-old male with pulmonary emphysema
  • Case 8: 38-year-old male with AIDS and headaches
  • Case 9: 25-year-old male with arm mass
  • Case 10: 57-year-old male with fatigue and left upper quadrant heaviness
  • Case 11: 62-year-old male with hepatosplenomegaly, skin lesions and cardiomyopathy
  • Case 12: 16-month-old infant with failure to thrive
  • Case 13: 36-year-old female with painless lower leg nodule
  • Case 14: 58-year-old female with several-year history of pelvic pain
  • Case 15: 52-year-old male with abdominal pain and bloody diarrhea
  • Case 16: 42-year-old female with tinnitus, hearing loss, and a mass at the cerebellopontine angle
  • Case 17: 52-year-old male with HIV with profuse, watery diarrhea
  • Case 18: 44-year-old male with history of progressive weakness and visual changes
  • Case 19: 20-year-old male with large, deep thigh mass
  • Case 20: 60-year-old male with multiple lung nodules
  • Case 21: 3-year-old boy with 7 cm abdominal mass
  • Case 22: 15-year-old male with large thigh mass
  • Case 23: 7-year-old male with headache, vomiting and gait ataxia